alkanes/alkenes

• what are alkanes?

  a group of saturated hydrocarbons
• what does saturated mean?

  only have single carbon-carbon bonds, no double bonds
• what is the general formula for alkanes?

  CnH2n+2
• alkanes are:

  • colourless
  • there is a gradual change in their physical properties as the number of carbon atoms in the chain increases
  • unreactive compounds
  • undergo combustion reaction
  • can be cracked into smaller more useful molecules
• what is the molecular formula of methane?

  CH4
• what is this: CH4?

  methane
• what is the molecular formula of ethane?

  C2H6
• what is this: C2H6?

  ethane
• what is the molecular formula for propane?

  C3H8
• what is this: C3H8?

  propane
• what is the molecular formula of butane?

  C4H10
• what is this: C4H10?

  butane
• what do alkanes always end in?

  -ane
• name the first 4 alkanes?

  methane, ethane, propane and butane
• what are the 2 substances that alkanes contain and why?

  hydrogen and carbon only because alkanes are hydrocarbons
• equation of combustion of methane?

  CH4 +2O2 --> CO2 + 2H20
• what does complete combustion form?

  water and carbon dioxide
• what are alkenes and why?

  unsaturated hydrocarbonds because they contain a double carbon bond which is shown as 2 lines between 2 of the carbon atoms e.g. C=C
• what do all alkenes contain?

  a double carbon bond
• what is the general formula for alkenes?

  CnH2n
• what is the molecular formula for ethene?

  C2H4
• what is the molecular formula for propene?

  C3H6
• what is the molecular formula for butene?

  C4H8
• what is the molecular formula for pentene?

  C5H10
• compounds that have a C=C double bond are also called unsaturated compounds.

  • this means they can make more bonds with other atoms by opening up the C=C bond and allowing incoming atoms to form another single bond with each carbon atom of the functional group
  • each of these carbon atoms now form 4 single bonds instead of 1 double and 2 single bonds
  • this makes them much more reactive than alkanes.
• what type of combustion to alkenes usually undergo?

  incomplete combustion because of the higher carbon to hydrogen ratio, producing a smoky flame in air.
• when does complete combustion occur in alkenes and give an example

  when there is excess oxygen so water and carbon dioxide form.

  butene + oxygen --> carbon dioxide + water
• when does incomplete combustion occur and give an example

  when there is not enough oxygen to burn so a mixture of products can form

  butene + oxygen --> carbon monoxide + water

  in addition to carbon monoxide, carbon in the form of soot can be made:

  butene + oxygen --> carbon + water
• when is carbon in the form of soot more likely to be made?

  more likely in higher alkenes with large number of carbons

  • this is seen as smokey yellow flames when the alkenes burn
• carbon-carbon double can break and form a single bond, allowing more atoms to attach to the carbon atoms

  A) =

  B) -

  
• what is hydrogenation?

  when alkenes react with hydrogen, forming an alkane
• when do hyrdogenation reactions occur?

  occur using a nickel catalyst
• hydrogen atoms add across the C=C in the hydrogenation of ethene to produce ethane.

  A) =

  B) H2

  C) H

  D) H

  
• what is halogenation?

  when a halogen reacts with the alkene, where the halogen atoms always add to the carbon atoms involved in the C=C double bond.
• bromine atoms add across the C=C in the addition reaction of ethene and bromine.

  A) Br2

  B) Br Br

  
• what is the difference between alkenes and alkanes?

  alkanes - saturated

  alkenes - unsaturated
• how can we tell alkanes and alkenes apart?

  the bromine water test
• the results of the test using bromine water with alkanes and alkenes.

  A) alkane

  B) alkene

  C) no change

  D) decolourised

  
• why does the bromine water stay orange when reacting with alkanes?

  because alkanes do not have double carbon bonds so the bromine remains in solution
• why does the bromine water decolourise?


  the bromine atoms add across the C=C bond, hence the solution no longer contains free bromine so it loses its colour